What does the 'then' in verse 16 refer to?
In verse 16, 'then' points to the specific crisis when the Antichrist enters the temple, defiles it, and immediately turns on the Jewish people. That defilement triggers Jesus' command for those in Judea to flee to the mountains without delay.
Les teaches that the 'then' in verse 16 is tied to the moment the word suddenly goes out that the Antichrist has come into the temple and defiled it. Since Israel will be back to animal sacrifices and the altar operating, he suggests the Antichrist may defile it in an especially offensive way, perhaps offering a hog, because that is anathema to a Jew. Whatever the exact act, the Antichrist will defile the temple and then immediately turn on the Jew. At that point Jesus says those in Judea must flee to the mountains. The following verses show this flight involves a mixed group, not one age or sex, and the urgency is so great that a person on the housetop should not go down to take anything out of his house.
Then let them which be in Judaea flee into the mountains: